In a composite digital video system, the video image is stored as a digital representation of the encoded color signal, where the color information is represented by an amplitude and phase modulated subcarrier. Generally, the signal is sampled at an integral multiple of the subcarrier frequency, e.g. 14.31818 MHz or four times the NTSC subcarrier frequency.
In a memory system based on the digital composite format, the sampling clock is phase-locked to the color burst present in the input signal. The digital samples are then placed sequentially in a memory device. If the entire waveform, including the color burst signal is then reconverted to analog form and sent to a receiver, proper color will result. If, however, the colorburst signal is not digitized into memory, it may be reconstructed on the fly during the reproducing process as may be the horizontal and vertical sync signals. This can save memory and also allows the device to produce a higher quality sync and burst waveform.
In order for this to be possible, the exact phase of the color reference signal must be determined before the image is placed in memory, since the colorburst signal and sync signals will not be recorded along with the image. The position of the vertical and horizontal synchronization pulses must also be unambiguously determined in order to properly generate memory addresses to store the image.
A particular problem is the relationship of the horizontal sync pulse to the color burst phase. Since the system clock must be based on the colorburst to insure correct color reproduction, the phase relationship of the system clock to the horizontal sync pulse is not known. While modern sync generation systems guarantee that the burst will be coherent, or in a constant phase relationship to the horizontal sync pulses, they do not necessarily have a predetermined relationship.
Thus, it is possible that the horizontal sync pulse will occur at exactly the same time as the system clock, causing the horizontal counter to be reset erratically, thus causing the image to jitter back and forth between two adjacent positions.
It is also necessary for the memory scanning counters in such a system to determine which of the four unique color fields are being written, so that when the image is read back from memory, the color information will be in the correct phase relationship to the synthesized colorburst signal.
It is therefore an object of this invention to lock a system clock generator to the input colorburst signal and also lock horizontal and vertical counters to the input sync signals in such a manner as to produce a stable image and exactly determine color phase and field number. In this process it is necessary to synchronize the system clock to the colorburst signal such that active transitions of the clock are in a known phase relationship to 0.degree. and 90.degree. points of the colorburst signal. The invention solves these problems by locking the horizontal counter to the to the external horizontal sync pulses such that the external pulse falls within a window that is three system clocks wide. If the pulse strays beyond the window, the counter is readjusted by two clock cycles to compensate. Thus, as long as the external horizontal sync pulse falls within the window, its exact relationship to the system clock is not important. Second, the external colorburst signal is sampled once every frame to determine which of the four unique color fields is being observed.
It is a further object of the invention to provide such a circuit that operates primarily in a digital format easily and cheaply implemented with integrated digital logic.
Other objects and features of the invention and the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings which exemplify the invention, it being understood that changes may be made in the specific method and apparatus disclosed herein without departing from the essentials of the invention set forth in the appended claims.